Drying of food
Practical Action
Answers
1 0.068kg per kilogram of dry air (find the intersection of the 60°C and 50% RH lines, and
then follow the chart horizontally right to read off the absolute humidity)
2 46.5°C (from the intersection of the 60°C and 50% RH lines, move left parallel to the wet-
bulb lines to read off the wet-bulb temperature)
3 20% (find the intersection of the 45°C and 75°C lines and follow the sloping RH line
upwards to read off the % RH)
4 36°C (find the intersection of the 55°C and 30% RH lines and follow the wet-bulb line left
until the RH reaches 100%)
5 50-10% (find the intersection of the 39°C wet-bulb and the 50°C dry-bulb temperatures,
and follow the horizontal line to the intersection with the 86°C dry-bulb line; read the
sloping RH line at each intersection (this represents the changes that take place when air is
heated prior to being blown over food))
6 10-70% (find the intersection of the 35°C wet-bulb and 70°C dry-bulb temperature, and
follow the wet-bulb line left until the intersection with the 40°C dry-bulb line; read sloping
RH line at each intersection (this represents the changes taking place as the air is used to
dry food; the air is cooled and becomes more humid as it picks up moisture from the food).
When food is placed in a dryer, there is a short period during which time the surface heats up.
This is followed by two distinct phases, the constant rate and the falling rate. In the constant
rate phase, water is removed from the surface of the food by evaporation. If the condition of the
air (the temperature and relative humidity) within the dryer is constant, the water is evaporated
at a constant rate. This is shown as the
steep straight line on the graph (figure 3).
As drying proceeds, water has to be removed
from the inside of the food. This becomes
more and more difficult as the water has to
travel further through the food from the
centre to the outside from where it is
evaporated. The drying rate slows down,
which is known as the falling rate period. On
the graph this is seen as the shallower part
of the curve. Eventually no more moisture
can be removed from the food and it is said
to be in equilibrium with the drying air (the
final part of the curve where it flattens out).
During the falling rate period, the rate of
drying is mainly controlled by the chemical
composition and physical structure of the
food. The temperature of the drying air is
also important during this phase as hot air
Figure 3: Drying rate
helps the moisture inside the food to move
towards the surface.
Measuring the drying rate
If a new type of dryer is to be used, or if a different type of food is to be dried, it is necessary to
carry out a few experiments to find the rate of drying. The information can then be used to find
the time that the food should spend in the dryer before the moisture content is low enough to
prevent spoilage by micro-organisms. The rate of drying also has an important effect on the
quality of the dried foods and (in artificial dryers) the fuel consumption. To find the rate of
drying you will need a clock or watch and a set of scales.
Weigh the food, place it in the dryer and leave for 5-10 minutes. Remove the food, reweigh it
and replace in the dryer. Continue like this until the weight of the food does not change any
more (it has reached a constant weight and all the moisture has been removed).
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